President Truman announced today that Charles G. Ross, 39-year-old Washington correspondent for the St. Louis Post-Dispatch, will become his Press Secretary on May 15. When Mr. Truman first took office, it was announced that J. Leonard Reinsch, Managing Director of three radio stations owned by James M. Cox, would act as secretary, but since then Mr. Cox has asked that Reinsch be allowed to return to his post with the stations because of the urgent need of his technical knowledge.